System and methods for performing neurophysiologic assessments during spine surgery

ABSTRACT

A system and methods for performing neurophysiology assessments during surgery, such as assessing the health of the spinal cord via at least one of MEP and SSEP monitoring and assessing bone integrity, nerve proximity, neuromuscular pathway, and nerve pathology during spine surgery.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention is an International Patent Application and claimsthe benefit of priority from commonly owned and co-pending U.S.Provisional Patent Application Ser. No. 60/649,724, entitled “System andMethods for Monitoring Before, During and/or After Surgery,” and filedon Feb. 2, 2005, the entire contents of which is hereby expresslyincorporated by reference into this disclosure as if set forth in itsentirety herein. Benefit is also claimed from commonly owned andco-pending U.S. Provisional Patent Application Ser. No. 60/719,897,entitled “Multi-Channel Stimulation Threshold Detection Algorithm forUse With Neurophysiology Monitoring Systems,” and filed on Sep. 22,2005, the entire contents of which is hereby expressly incorporated byreference into this disclosure as if set forth in its entirety herein.

BACKGROUND OF THE INVENTION

I. Field of the Invention

The present invention relates generally to a system and methods forperforming neurophysiologic assessments during surgery, such asassessing the health of the spinal cord via at least one of MEP and SSEPmonitoring, and assessing at least one of bone integrity, nerveproximity, neuromuscular pathway, and nerve pathology (free-run andevoked) during spine surgery.

II. Discussion of the Prior Art

Surgical procedures conducted on or around the spine can be beneficialin reversing or mitigating a variety of ailments commonly suffered bypatients. Despite ongoing advances in surgical methods, however,neurological impairment remains a serious concern during surgical spineprocedures. The safety of the spinal cord is of paramount importancebecause damage to the spinal cord may have devastating results for thepatient. Consequences of spinal cord damage may range from a slight lossof sensation to complete paralysis of the extremities, depending on thelocation and extent of damage. Assessing the spinal cord before, duringand/or after surgery can provide the surgeon with valuable informationon the health of the cord. Such information may allow the surgeon toinitiate corrective measures if the health of the cord is compromised,thereby decreasing the chance of permanent spinal cord damage and theresulting consequences.

The spinal cord is composed of a number of nerve pathways includingmotor and sensory pathways. Motor pathways transmit signals from thebrain to the various muscle groups of the body. Conversely, sensorypathways transmit signals from the skin and other parts of the body upto the brain. Currently, methods exist for assessing the health of thespinal cord by monitoring electrical transmission along these pathways.Degradation of an electrical signal introduced near the origin of apathway and monitored near the end of the pathway is indicative ofdamage to the spinal cord.

Motor pathway monitoring may be accomplished by stimulating the motorcortex in the brain and recording the resulting EMG response of variousmuscles in the upper and lower extremities. This method is referred toas trans-cranial electrical motor evoked potential (tc_(e) MEP, orsimply “MEP”) monitoring.

Sensory pathway monitoring may be accomplished by stimulating aperipheral nerve that enters the spinal cord below the level of surgeryand recording the resulting action potentials from electrodes on thescalp or high level cervical vertebra. This method is referred to assomatosensory evoked potential (SSEP) monitoring.

While MEP and SSEP monitoring are generally effective for assessing thehealth of the spinal cord, data from the current methods is typicallyreceived as electrical waveforms that must first be analyzed andinterpreted in order to provide meaningful data to the surgeon.Interpreting the data can be a complex and difficult task and typicallyrequires specially trained personnel to complete it. This isdisadvantageous in that it increases surgery time (additional timeneeded to interpret data and communicate significance to the surgeon),translates into extra expense (having extra highly trained persons inattendance), and oftentimes presents scheduling challenges because mosthospitals do not retain such specially trained personnel.

Based on the foregoing, a need exists for a better system and methodsfor monitoring the health of the spinal cord before, during, and orafter surgery, and in particular, a need for a system that has theability to conduct MEP and SSEP monitoring while quickly presenting datato the user in a simplified yet meaningful way. A need also exists for asystem for monitoring the health of the spinal cord while providing theability to assess at least one of bone integrity, nerve proximity,neuromuscular pathway, and nerve pathology (free-run and evoked) duringspine surgery.

The present invention is directed at addressing the above identifiedneeds and overcoming, or at least improving upon, the disadvantages ofthe prior art.

SUMMARY OF THE INVENTION

The present invention includes a system and related methods forperforming neurophysiologic assessments during surgery, such asassessing the health of the spinal cord via at least one of MEP and SSEPmonitoring, and assessing at least one of bone integrity, nerveproximity, neuromuscular pathway, and nerve pathology (free-run andevoked) during spine surgery.

According to a broad aspect, the present invention includes a surgicalsystem, comprising a control unit and a surgical instrument. The controlunit has at least one of computer programmed software, firmware andhardware capable of delivering a stimulation signal, receiving andprocessing neuromuscular or other bioelectric responses due to thestimulation signal, and identifying a relationship between theneuromuscular response and the stimulation signal. The surgicalinstrument has at least one stimulation electrode in communication withthe control unit (via hardwire or wireless) for transmitting astimulation signal. The control unit is capable of assessing at leastone of spinal cord health via MEP or SSEP monitoring, bone integrity,nerve proximity, and nerve pathology based on the identifiedrelationship between a stimulation signal and a correspondingneuromuscular response.

In a further embodiment of the surgical system of the present invention,the control unit is further equipped to communicate at least one ofalpha-numeric and graphical information to a user regarding at least oneof MEP, SSEP, bone integrity, nerve proximity, nerve direction, andnerve pathology.

In a further embodiment of the surgical system of the present invention,the hardware employed by the control unit to provide a stimulationsignal may comprise an MEP stimulator capable of delivering a range ofhigh voltage, constant current pulses for stimulating the motor cortexthrough the skull, wherein the control unit assesses the health of thespinal cord based on the identified relationship between theneuromuscular response and the stimulation signal.

In a further embodiment of the surgical system of the present invention,the MEP stimulator may be communicatively linked to the control unit viawireless technology.

In a further embodiment of the surgical system of the present invention,a bite block may be used in conjunction with the MEP stimulator, whereinthe bite block is communicatively linked to the system and placement ofthe bite block may be confirmed by the system prior to MEP stimulation.

In a further embodiment of the present invention, the hardware employedby the control unit to provide a stimulation signal may comprise apatient module capable of delivering a range of low voltage, constantcurrent pulses for stimulating a peripheral nerve, wherein the controlunit assess the health of the nerve pathways based on the identifiedrelationship between the stimulation signal and the correspondingneuromuscular response.

In a further embodiment of the present invention, the hardware employedby the control unit to provide a stimulation signal may comprise apatient module capable of delivering a range of low voltage pulses at aconstant current (or constant voltage, if desired) for stimulating anerve, wherein the control unit determines at least one of boneintegrity, nerve proximity, nerve direction, and nerve pathology basedon the identified relationship between the neuromuscular response andthe stimulation signal.

In a further embodiment of the surgical system of the present invention,the surgical instrument may comprise at least one of a device forforming a hole in bone (e.g. for testing pedicle integrity), a devicefor accessing a surgical target site, and a device for maintainingcontact with a nerve during surgery.

In a further embodiment of the surgical system of the present invention,the surgical instrument comprises a screw test instrument, wherein thecontrol unit determines the degree of electrical communication betweenthe screw test instrument and an exiting spinal nerve root to assesswhether a pedicle has been breached during at least one of pilot holeformation (e.g. via an awl), pilot hole preparation (e.g. via a tap),and screw placement (e.g. via a ball-tipped probe).

In a further embodiment of the surgical system of the present invention,the surgical instrument comprises a nerve root retractor, wherein thecontrol unit determines nerve pathology based on the identifiedrelationship or change in relationship between the neuromuscularresponse and the stimulation signal.

In a further embodiment of the surgical system of the present invention,the surgical instrument comprises a dilating instrument, wherein thecontrol unit determines at least one of proximity and direction betweena nerve and the instrument based on the identified relationship betweenthe neuromuscular response and the stimulation signal.

In a further embodiment of the surgical system of the present invention,the dilating instrument comprises at least one of a K-wire, anobturator, a dilating cannula, and a working cannula.

In a further embodiment of the surgical system of the present invention,the surgical instrument comprises a tissue retractor assembly and thecontrol unit determines at least one of proximity and direction betweena nerve and the instrument based on the identified relationship betweenthe neuromuscular response and the stimulation signal.

BRIEF DESCRIPTION OF THE DRAWINGS

Many advantages of the present invention will be apparent to thoseskilled in the art with a reading of this specification in conjunctionwith the attached drawings, wherein like reference numerals are appliedto like elements and wherein:

FIG. 1 is a perspective view of an exemplary surgical system 10 capableof conducting multiple nerve and spinal cord monitoring functionsincluding but not necessarily limited to MEP, SSEP, neuromuscularpathway, bone integrity, nerve detection, and nerve pathology (evoked orfree-run EMG) assessments;

FIG. 2 is a block diagram of the surgical system 10 shown in FIG. 1;

FIG. 3 is an exemplary screen display illustrating a site selectionscreen for indicating the spinal region to be monitored;

FIG. 4 is an exemplary screen display illustrating a drop down functionmenu for navigating between different functions of the system 10 duringcervical and thoracolumbar procedures;

FIG. 5 is an exemplary screen display illustrating another embodimentdrop down function menu for navigating between different functions ofthe system 10 during lumbar procedures;

FIG. 6 is an exemplary screen display illustrating a function displayscreen from which the function menu may be accessed;

FIG. 7 is exemplary screen display illustrating the function displayscreen of FIG. 6 in which the function menu has been opened;

FIG. 8 is an exemplary screen display illustrating one embodiment of ageneral system setup screen;

FIG. 9 is a graph illustrating a plot of a stimulation current signalcomprising a train of pulses capable of producing a neuromuscularresponse (EMG) of the type shown in FIG. 10;

FIG. 10 is a graph illustrating a plot of the neuromuscular response(EMG) of a given myotome over time based on a stimulation signal (suchas shown in FIG. 9) applied to the motor cortex and transmitted to anerve bundle coupled to the given myotome;

FIG. 11 is a front perspective view of an MEP stimulator capable ofdelivering high current stimulation signals necessary top elicit MEPresponses;

FIG. 12 is a view of the back of the MEP stimulator of FIG. 11;

FIG. 13 is a perspective view of a bite block for use with the presentinvention;

FIG. 14 is a perspective view of the bite block of FIG. 13 including apair of electrodes communicatively linked to the system as a means ofproviding a safety check;

FIG. 15 is a perspective view of the bite block of FIG. 13 including oneelectrode communicatively linked to the system as a means of providing asafety check;

FIG. 16 is a graph illustrating a plot of EMG response peak-to-peakvoltage (Vpp) for each given stimulation current level (I_(Stim))forming a stimulation current pulse according to the present invention(otherwise known as a “recruitment curve”);

FIGS. 17A-17D are graphs illustrating the fundamental steps of a rapidcurrent threshold-hunting algorithm according to one embodiment of thepresent invention;

FIG. 18 is a flowchart illustrating the method by which a multi-channelhunting algorithm determines whether to perform or omit a stimulation;

FIGS. 19A-19C are graphs illustrating use of the threshold huntingalgorithm of FIG. 17 and further omitting stimulations when the likelyresult is already clear from previous data;

FIG. 20 is a flowchart illustrating the sequence employed by thealgorithm to determine and monitor I_(thresh);

FIG. 21 is a graph illustrating the confirmation step employed by thealgorithm to determine whether I_(thresh) has changed from a previousdetermination;

FIG. 22 is an exemplary screen display illustrating one embodiment of aMEP automatic mode setup screen according to the present invention;

FIGS. 23-24 are exemplary screen displays illustrating variousembodiments of the MEP Automatic mode function according to one aspectof;

FIG. 25 is an exemplary screen display illustrating one embodiment of aMEP manual mode setup screen according to the present invention;

FIGS. 26-27 are exemplary screen displays illustrating variousembodiments of the MEP manual mode function according to one aspect ofthe present invention;

FIG. 28 is a graph illustrating a decrease in the EMG response amplitudedemonstrating an additional method of assessing the health of the spinalcord and nerve pathways;

FIG. 29 is a graph illustrating an increase in the EMG response latencyperiod demonstrating another method of assessing the health of thespinal cord;

FIG. 30 is a graph illustrating a change in EMG response waveformmorphology from a complex multiphasic waveform to a bi-phasic waveformdemonstrating yet one more additional method for assessing the health ofthe spinal cord;

FIG. 31 is an exemplary screen display illustrating one embodiment ofthe SSEP function of the present invention;

FIG. 32 is an exemplary screen display illustrating one embodiment of aSSEP setup screen according to one aspect of the present invention;

FIG. 33 is an exemplary screen display illustrating a method ofgenerating a surgical report according to one embodiment of the presentinvention;

FIG. 34 is an exemplary screen display illustrating a method ofselecting between a full surgical report or a summary surgical reportaccording to one embodiment of the present invention;

FIGS. 35A-35C is an exemplary representation of a summary reportaccording to one embodiment of the present invention; and

FIGS. 36A-36E is an exemplary representation of a full report accordingto one embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Illustrative embodiments of the invention are described below. In theinterest of clarity, not all features of an actual implementation aredescribed in this specification. It will of course be appreciated thatin the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis disclosure. The systems disclosed herein boast a variety ofinventive features and components that warrant patent protection, bothindividually and in combination.

FIG. 1 illustrates, by way of example only, a surgical system 10 capableof assessing the health of the spinal cord via at least one of MEP andSSEP monitoring and quickly presenting meaningful data to the surgeon.The surgical system 10 is further capable of conducting other neuralmonitoring functions including, but not necessarily limited to,stimulated EMG, neuromuscular pathway assessments (Twitch Test), pediclescrew testing (Screw Test), nerve proximity monitoring (Detection), andnerve pathology monitoring (Nerve Retractor). It is expressly notedthat, although described herein largely in terms of use in spinalsurgery, the surgical system 10 and related methods of the presentinvention are suitable for use in any number of additional procedures,surgical or otherwise, wherein assessing the health of the spinal cordand/or various other nerves may prove beneficial, such as, for exampleonly, when the blood supply to the spinal cord is at risk duringthoracic vascular surgery.

The surgical system 10 includes a control unit 12, a patient module 14,an EMG harness 16, including eight pairs of EMG electrodes 18 and areturn (anode) electrode 22 coupled to the patient module 14, an MEPstimulator 21, a pair of peripheral nerve stimulation (PNS) electrodes25 also coupled to the patient module 14, at least one pair ofstimulation electrodes 23 coupled to the MEP stimulator 21, and a hostof surgical accessories 32 capable of being coupled to the patientmodule 14 via one or more accessory cables 30. The surgical accessories32 may include, but are not necessarily limited to, stimulationaccessories (such as a screw test probe 36 and dynamic stimulation clips42, 52), surgical access components (such as a K-wire 62, one or moredilating cannula 64, a working cannula 66, and a tissue retractionassembly 70), and neural pathology monitoring devices (such as a nerveroot retractor 76).

FIG. 2 is a block diagram of the surgical system 10, the operation ofwhich will be explained in conjunction with FIG. 1. The control unit 12includes a touch screen display 26 and a base 28, which collectivelycontain the essential processing capabilities for controlling thesurgical system 10. The touch screen display 26 is preferably equippedwith a graphical user interface (GUI) capable of graphicallycommunicating information to the user and receiving instructions fromthe user. The base 28 contains computer hardware and software thatcommands the stimulation sources (e.g. MEP stimulator 21 and patientmodule 14) receives digital and/or analog signals and other informationfrom the patient module 14, processes the EMG responses, and displaysthe processed data to the operator via the display 26. The primaryfunctions of the software within the control unit 12 include receivinguser commands via the touch screen display 26, activating stimulation inthe requested mode (MEP, SSEP, Twitch Test, Screw Test (Basic,Difference, Dynamic), Detection, and Nerve Retractor), processing signaldata according to defined algorithms (described below), displayingreceived parameters and processed data, and monitoring system status.

The patient module 14 is connected via a data cable 24 to the controlunit 12, and contains the electrical connections to electrodes, signalconditioning circuitry, stimulator drive and steering circuitry, and adigital communications interface to the control unit 12. In use, thecontrol unit 12 is situated outside but close to the surgical field(such as on a cart adjacent the operating table) such that the display26 is directed towards the surgeon for easy visualization. The patientmodule 14 may be located near the patient's legs or may be affixed tothe end of the operating table at mid-leg level using a bedrail clamp.The position selected should be such that all EMG electrodes 18 canreach their farthest desired location without tension during thesurgical procedure.

The information displayed to the user on the display 26 may include, butis not necessarily limited to, alpha-numeric and/or graphicalinformation regarding any of the requested modes (e.g., MEP, SSEP,Twitch Test, Screw Test (Basic, Difference, Dynamic), Detection, andNerve Retractor), myotome/EMG levels, stimulation levels, etc . . . Inone embodiment, set forth by way of example only, this information mayinclude at least some of the following components (depending on theactive mode) as set forth in Table 1:

TABLE 1 Screen Component Description Spine Image An image of the humanbody/skeleton showing the electrode placement on the body, with labeledchannel number tabs on each side (1-4 on the left and right). Left andright labels will show the patient orientation. The channel number tabsmay be highlighted or colored depending on the specific function beingperformed. Myotome & Level A label to indicate the Myotome name andcorresponding Spinal Names Level(s) associated with the channel ofinterest. Menu A drop down navigation component for toggling betweenfunctions. Display Area Shows procedure-specific information includingstimulation results. Color Indication Enhances stimulation results witha color display of green, yellow, or red corresponding to the relativesafety level determined by the system. Mode Indicator Graphics and/orname to indicate the currently active mode (MEP, SSEP, Twitch Test,Basic Screw Test, Dynamic Screw Test, Difference Screw Test, Detection,Nerve Retractor). In an alternate embodiment, Graphics and/or name mayalso be displayed to indicate the instrument in use, such as thedilator, K-wire, retractor blades, screw test instruments, andassociated size information, if applicable, of the cannula, with thenumeric size. If no instrument is in use, then no indicator isdisplayed. Stimulation Bar A graphical stimulation indicator depictingthe present stimulation status (ie.. on or off and stimulation currentlevel) Sequence Bar Shows the last several stimulation results andprovides for annotation of results. EMG waveforms EMG waveforms may beoptionally displayed on screen along with the stimulation results.

Control of the surgical system 10 is, according to one embodiment,performed by user selection of available options on the GUI display 26,which will now be described (by way of example only) with reference toFIGS. 3-6. FIG. 3 is an exemplary display of a “Site Selection” screenfrom which a user may initially select the spinal region (i.e. Cervical,Thoracolumbar, or Lumbar) in which the procedure is to be performed bytouching one of the site selection tabs 120. EMG electrode placementdiffers for each spinal region, based on the specific spinal nerves ofthat region and the associated muscle myotomes. Upon selection of aparticular spinal region, each EMG channel is labeled with a myotomeaccording to a preferred EMG configuration for that spinal region.

The Site Selection screen preferably sets forth a list of the modes 122available for each spinal region. By way of example only, the Cervicaland Thoracolumbar spinal regions may include the Twitch Test, BasicScrew Test, Difference Screw Test, Dynamic Screw Test, MEP Auto, and MEPManual modes, while the Lumbar spinal region includes the Twitch Test,Basic Screw Test, Difference Screw Test, Dynamic Screw Test, MaXcess®Detection, and Nerve Retractor modes, all of which will be described ingreater detail below. (Although not shown, each of the spinal regionsmay also include an SSEP mode, as will be described in greater detailbelow.) The Twitch Test mode is designed to assess the neuromuscularpathway via the so-called “train-of-four” test to ensure theneuromuscular pathway is free from muscle relaxants prior to performingneurophysiology-based testing, such as bone integrity (e.g. pedicle)testing, nerve detection, and nerve retraction. This is described ingreater detail within Int'l Patent App. No. PCT/US05/36089, entitled“System and Methods for Assessing the Neuromuscular Pathway Prior toNerve Testing,” filed Oct. 7, 2005, the entire contents of which ishereby incorporated by reference as if set forth fully herein. The BasicScrew Test, Difference Screw Test, and Dynamic Screw Test modes aredesigned to assess the integrity of bone (e.g. the pedicle) during allaspects of pilot hole formation (e.g., via an awl), pilot holepreparation (e.g. via a tap), and screw introduction (during and after).These modes are described in greater detail in Int'l Patent App. No.PCT/USO2/35047 entitled “System and Methods for Performing PercutaneousPedicle Integrity Assessments,” filed on Oct. 30, 2002, andPCT/US2004/025550, entitled “System and Methods for Performing DynamicPedicle Integrity Assessments,” filed on Aug. 5, 2004 the entirecontents of which are both hereby incorporated by reference as if setforth fully herein. The MaXcess® Detection mode is designed to detectthe presence of nerves during the use of the various surgical accessinstruments of the surgical system 10, including the k-wire 62, dilator64, cannula 66, retractor assembly 70. This mode is described in greaterdetail within Int'l Patent App. No PCT/US02/22247, entitled “System andMethods for Determining Nerve Proximity, Direction, and Pathology DuringSurgery,” filed on Jul. 11, 2002, the entire contents of which is herebyincorporated by reference as if set forth fully herein. The NerveRetractor mode is designed to assess the health or pathology of a nervebefore, during, and after retraction of the nerve during a surgicalprocedure. This mode is described in greater detail within Int'l PatentApp. No. PCT/USO2/30617, entitled “System and Methods for PerformingSurgical Procedures and Assessments,” filed on Sep. 25, 2002, the entirecontents of which is hereby incorporated by reference as if set forthfully herein. The MEP Auto and MEP Manual modes are designed to test themotor pathway to detect potential damage to the spinal cord bystimulating the motor cortex in the brain and recording the resultingEMG response of various muscles in the upper and lower extremities.These modes will be described in greater detail below.

Before addressing the MEP and SSEP functionality of the surgical system10 of the present invention, various general features of the surgicalsystem 10 will be explained. In one embodiment, the surgical system 10provides the ability to quickly and easily switch or toggle back andforth between different modes during a surgical procedure. Togglingbetween the various functions of the surgical system 10 may preferablybe accomplished by selecting from a drop down mode menu 104, asillustrated in FIGS. 4-5. Upon initial site selection, the mode menu 104is available on a “menu screen.” FIG. 4 represents a menu screen for thecervical and thoracolumbar spinal regions and FIG. 5 represents a menuscreen for the lumbar spinal region. The menu screen includes the modemenu 104 and may optionally also include instructions for using andrecalling the mode menu 104 at a later time. The mode menu 104 may berecalled directly from the display screen for any given mode, such asthe exemplary “MEP Automatic” mode illustrated in FIG. 6-7. Selectingthe menu button 118 labeled (by way of example only) “Select Mode”expands and the drop-down mode menu 104 as seen in FIG. 7. Using themenu 104, the surgeon or other qualified user may open any of thefunctions by selecting the function tab 110 corresponding to the desiredfunction. Also from the menu 104, the user may optionally select to viewactual EMG waveforms 116 in addition to the other stimulation results.This is accomplished by selecting an EMG tab (entitled “with EMG”)corresponding to the desired function. It should be understood that thedrop down menu described above is only a preferred method of navigatingbetween functions and any of a number of different methods may be used.By way of example only, a menu bar containing the different functionbuttons may be constantly displayed across the top or bottom of thescreen.

The surgical system 10 also includes a Setup mode that provides a simplemeans for selecting and/or changing various options and parametersassociated with the surgical system 10 and each the modes. In oneembodiment, the display screen for each mode includes a setup tab thatallows a user to access and modify the parameters for any or all of themodes. FIG. 8 is an exemplary illustration, set forth by way of exampleonly, of a general system setup screen. From the system set up screen,the user may adjust the system volume, adjust the free-run EMG volume,change the EMG scale, turn the various EMG channels on and off, set thedate and time, conduct an impedance test to check the electricalconnection between the EMG electrodes and the patient's skin, andshutdown the system 10. The setup options relating to each of the modes(including MEP and SSEP) will be discussed in greater detail below.

The neuromonitoring functionality of the surgical system 10 (exceptSSEP, which will be described in detail below) is based on assessing theevoked response of the various muscles myotomes monitored by thesurgical system 10 in relation to a stimulation signal transmitted bythe system 10. This is best shown in FIG. 9-10, wherein FIG. 10illustrates the resulting EMG of a monitored myotome in response to eachpulse of the stimulation signal shown in FIG. 9. The EMG responsesprovide a quantitative measure of the nerve depolarization caused by theelectrical stimulus. In one embodiment, EMG response monitoring isaccomplished via 8 pairs EMG electrodes 18 (placed on the skin over themuscle groups to be monitored), a common electrode 20 providing a groundreference to pre-amplifiers in the patient module 14, and an anodeelectrode 22 providing a return path for the stimulation current. Apreferred EMG electrode for use with the system 10 is the dual surfaceelectrode which is shown and described in detail in the commonly ownedand co-pending U.S. patent application Ser. No. 11/048,404, entitled“Improved Electrode System and Related Methods,” filed on Jan. 31, 2005,which is expressly incorporated by reference into this disclosure as ifset forth in its entirety herein. It should be appreciated however, thatany of a variety of known electrodes can be employed, including but notlimited to surface pad electrodes and needle electrodes. It should alsobe appreciated that EMG electrode placement depends on a multitude offactors, including for example, the spinal cord level and particularnerves at risk and user preference, among others. Though not essential,electrode placement is preferably undertaken to correspond with thepreferred EMG configuration determined during site selection. In oneembodiment (set forth by way of example only), the preferred EMGconfiguration is described for Lumbar in Table 2, Thoracolumbar in Table3, and Cervical in Table 4 below:

TABLE 2 Lumbar Color Channel Myotome Nerve Spinal Level Red- Right 1Right Vastus Medialis Femoral L2, L3, L4 Orange Right 2 Right TibialisAnterior Common L4, L5 Peroneal Yellow Right 3 Right Biceps FemorisSciatic L5, S1, S2 Green Right 4 Right Medial Gastroc. Post Tibial S1,S2 Blue Left 1 Left Vastus Medialis Femoral L2, L3, L4 Violet Left 2Left Tibialis Anterior Common L4, L5 Peroneal Gray Left 3 Left BicepsFemoris Sciatic L5, S1, S2 White Left 4 Left Medial Gastroc. Post TibialS1, S2

TABLE 3 Thoracolumbar Color Channel Myotome Nerve Spinal Level Red Right1 Right Abductor Median C6, C7, C8, T1 Pollicis Brevis Orange Right 2Right Vastus Medialis Femoral L2, L3, L4 Yellow Right 3 Right TibialisAnterior Common L4, L5 Peroneal Green Right 4 Right Abductor HallucisTibial L4, L5, S1 Blue Left 1 Left Abductor Median C6, C7, C8, T1Pollicis Brevis Violet Left 2 Left Vastus Medialis Femoral L2, L3, L4Gray Left 3 Left Tibialis Anterior Common L4, L5 Peroneal White Left 4Left Abductor Hallucis Tibial L4, L5, S1

TABLE 4 Cervical Color Channel Myotome Nerve Spinal Level Red Right 1Right Deltoid Axilliary C5, C6 Orange Right 2 Right Flexor CarpiRadialis Median C6, C7, C8 Yellow Right 3 Right Abductor Median C6, C7,C8, Pollicis Brevis T1 Green Right 4 Right Abductor Hallucis Tibial L4,L5, S1 Blue Left 1 Left Deltoid Axillary C5, C6 Violet Left 2 LeftFlexor Carpi Radialis Median C6, C7, C8 Gray Left 3 Left Abductor MedianC6, C7, C8, Pollicis Brevis T1 White Left 4 Left Abductor HallucisTibial L4, L5, S1

In a first broad aspect of the present invention, the surgical system 10is capable of assessing the health of the spinal cord via MEPmonitoring. The surgical system 10 performs the MEP function bytransmitting electrical stimulation signals from the MEP stimulator 21through the motor cortex of the brain. The stimulation signals createaction potentials which travel along the spinal cord and into exitingnerve roots, evoking activity from muscles innervated by the nerves.Evoked EMG responses of the muscles are recorded by the system 10 andanalyzed in relation to the stimulation signal (discussed below).Resulting data from the analysis is conveyed to the surgeon on the GUIdisplay 26.

MEP stimulation signals are generated in the MEP stimulator 21 anddelivered to the motor cortex via stimulation electrodes 23 connected tothe MEP stimulator 21. Stimulation electrodes 23 may take the form ofneedle, corkscrew, or surface pad electrodes, among other known forms.Typically a pair of stimulation electrodes 23, one cathode and oneanode, are placed on opposite sides of the skull to transcraniallystimulate the motor cortex. In a preferred embodiment, a single MEPstimulation signal includes multiple electrical pulses deliveredtogether as one group or train. Stimulation signals comprising multiplepulses are desirable when stimulating the motor cortex, such as whenperforming MEP, because a more reliable response can be generated as aresult. Each individual pulse of the stimulation signal will causedepolarization (provided the current level is greater than or equal tothe stimulation threshold). Each train of pulses (i.e. each individualstimulation signal) is preferably delivered as a series of rectangularmonophasic pulses, such as that illustrated, by way of example only, inFIG. 9. The stimulation signal includes a predetermined number of pulses(N) separated by an interpulse gap (G) measured from leading edge toleading edge, each pulse having a pulse width (W) and a current level(I). In one embodiment, the current (I), pulse width (W), and interpulsegap (G) remain constant within a given stimulation signal. By way ofexample only, MEP stimulator 21 may deliver a stimulation signalcomprising four pulses (N=4) having a pulse width of 50 μs (W=50 μs), acurrent level of 250 mA (I=250 mA), and separated by 2 ms each (G=2 ms).It will be appreciated, however, that these parameters are set forth byway of example only and that any or all of these parameters may bemodified without departing from the scope of the present invention.

FIGS. 11 and 12 illustrate front and back views of an example embodimentof MEP stimulator 21, respectively. MEP stimulator 21 includes a highvoltage transformer and signal conditioning circuitry (not shown), acommunications link to the control unit 12, electrical connections 27for stimulation electrodes 23, power source connection 35, power switch33 and LED lights 29 for indicating the type of connection (e.g.wireless Bluetooth connection or USB data cable), whether the power ison, and whether the stimulator 21 is ready to stimulate. In a preferredembodiment, set forth by way of example only, MEP stimulator 21 iscurrent controlled and may deliver stimulation pulses with a currentlevel (I) ranging from 0 mA to 1000 mA. To generate the desired current,stimulator 21 has a voltage output ranging from 0V to 1000V. Stimulationsignals generated may include a train of pulses (N) ranging in numberfrom 1 to 8. Pulses may be separated by an interpulse gap (G) rangingfrom 1 ms to 10 ms and pulse widths (W) may range from 50 μs to 400 μs.MEP stimulator 21 is further capable of delivering either a positivepulse or a negative pulse and may do so automatically or upon manualselection. Additionally, MEP stimulator 21 may have more than onestimulation channel, thus, additional pairs of stimulation electrodes 23may be arranged on the skull. This may be advantageous in that theeffectiveness of a stimulation pulse originating from one position onthe skull may vary between different recording sites.

MEP stimulator 21 is communicatively linked to the control unit 12 whichcommands the stimulator 21 to deliver stimulation signals (according tothe predetermined parameters) at the proper time. MEP stimulator 21 maybe linked to the control unit 12 with a data cable that connects in anysuitable manner or protocol, including but not limited to a USB cablethat plugs into a USB port 31 on MEP stimulator 21 and control unit 12.Alternatively, MEP stimulator 21 may be linked to the control unit 12via wireless technology. By way of example only, this may beaccomplished by providing each of the control unit 12 and the MEPstimulator 21 with Bluetooth transceivers, which are commerciallyavailable and commonly known in the prior art, allowing the control unit12 to transmit stimulation commands to the MEP stimulator 21 via arobust radio link. In use, this provides flexibility in positioning theMEP stimulator 21 in relation to the control unit 12, as well asreducing the number of wires and connections required for setup. The MEPstimulator 21 may be positioned outside the sterile area but should belocated such that the stimulation electrodes 23, attached to thestimulator 21, may be positioned on the patient's head without tension.By way of example only, MEP stimulator 21 may be placed on the surgicaltable adjacent to the patient's head. Optionally, the MEP stimulator 21may be fashioned with a mount or hook (not shown) and hung from thesurgical table, an IV pole near the patient's head, or other equipmentpositioned near the patient.

According to one embodiment, MEP stimulator 21 may be optionallyprovided with a “bite block” 130 (FIGS. 13-15) to protect the patient'steeth, tongue, and cheeks from potential damage that may be caused by areflexive clenching of the jaws (bite reflex) which may occur during MEPstimulation. Bite block 130 may be embodied in any of a variety ofdevices that can be inserted between the upper and lower teeth,absorbing the force of a bite reflex and preventing the tongue and/orcheeks from being bitten. By way of example only, bite block 130 maycomprise a generally U shape matching the curvature of the mouth. A bitechannel 132 may be provided to encourage proper positioning in themouth. Preferably, bite block 130 may be disposable and a new bite blockprovided for each patient. As shown in FIGS. 14-15, one or moreelectrodes 134 may be incorporated into the bite block 130. By employingelectrodes 134 or other moisture sensing features, the system 10 mayconduct a safety check and confirm that bite block 130 is properlypositioned prior to initiating an MEP stimulation pulse that may resultin a bite reflex.

In the embodiment of FIG. 14, two electrodes 134 (one positive (anode)and one negative (cathode)) may be positioned near the distal ends ofbite block 130 such that a gap, indicated by line X, exists between theelectrodes 134. Attachable cables 136 may communicatively link theelectrodes 134 to patient module 14. Alternatively, the electrodes 134may be connected to MEP stimulator 21 via attachable cables 136. Thesurgical system 10 confirms bite block 130 is in position by applying asmall electrical current, via patient module 14 or MEP stimulator 21, tothe cathode electrode and measuring the impedance between the cathodeand anode. Positioning bite block 130 in the mouth moistens electrodes134 which results in a decrease in the impedance value. Conversely, ifthe bite block 130 is not positioned in the mouth, the electrodes 134may not be moistened and the impedance value will be substantiallyhigher.

In the embodiment of FIG. 15, one electrode 134 may be incorporated intobite block 130 and connected to the MEP stimulator via attachable cable136. Electrode 134 may be employed in conjunction with stimulationelectrode 23 to conduct the safety check and confirm bite block 130 isin position prior to initiating MEP stimulation. The system 10 applies asmall electrical current to the stimulation electrode 23 located on thehead and the impedance between the stimulation electrode 23 and the biteblock electrode 134 is determined. If the impedance value is not withina predetermined safe range, it may be an indication that the bite block130 is not positioned properly thus decreasing its effectiveness. Thebite block 130 may be repositioned and tested again until the impedancetest indicates proper positioning.

In a still further embodiment, electrode 134 of bite block 130 maycomprise one of MEP stimulation electrodes 23. Electrode 134 isconnected, via an attachable cable 136, to MEP stimulator 21 on the samestimulation channel as the corresponding stimulation electrode 23. Inthis manner, transcranial stimulation of the motor cortex is achieved bysending an MEP stimulation pulse from an electrode on the top of thehead to an electrode located in the mouth, as opposed to sending thestimulation signal from one side of the head to the other. In additionto providing an alternate path for the stimulation pulse, utilizing thebite block 130 as a part of the stimulation circuit ensures that theprotective bite block 130 is in position prior to MEP stimulation.Additionally, the impedance tests described above may be implementedthrough this embodiment.

Although bite block 130 has been described above with reference to thesurgical system 10, it will be appreciated as within the scope of theinvention to use bite block 130 in conjunction with any device or systemused for MEP stimulation. It will be further appreciated as notdeparting from the scope of the invention that bite block 130 may beimplemented as an independent system utilizing its own electrical sourceor the electrical source of any system it may be used in conjunctionwith, for any of a variety of situations where confirming placement of abite block may be beneficial.

A basic premise underlying the methods employed by the system 10 for MEPmonitoring (as well as the other nerve monitoring functions conducted bysystem 10) is that neurons and nerves have characteristic thresholdcurrent levels (I_(Thresh)) at which they will depolarize, resulting indetectable muscle activity. Below this threshold current, stimulationsignals will not evoke a significant EMG response. Each EMG response canbe characterized by a peak-to-peak voltage of V_(pp)=V_(max)−V_(min),shown in FIG. 10. Once the stimulation threshold (I_(Thresh)) isreached, the evoked response is reproducible and increases withincreasing stimulation until saturation is reached as shown in FIG. 16.This is known as a “recruitment curve.” In one embodiment, a significantEMG response is defined as having a V_(pp) of approximately 100 uV. Thelowest stimulation signal current that evokes this threshold voltage(V_(Thresh)) is called I_(Thresh). I_(thresh) increases as the degree ofelectrical communication between a stimulation signal and a nervedecreases and conversely, I_(thresh) decreases as the electricalcommunication increases between the nerve and stimulation pulse. Thusmonitoring I_(thresh) during MEP can provide the surgeon with usefulinformation about the health of the spinal cord. For example, ifI_(thresh) is too high or increases from a previous measurement, it mayindicate a problem in the spinal cord inhibiting transmission(communication) of the stimulation signal to the nerve. I_(thresh) canbe conveyed as a simple numerical value, thereby providing the surgeonwith simple, comprehensible data from the MEP test, without the need forseparate analysis by other highly trained personnel. CalculatingI_(thresh) also provides valuable information for other nerve monitoringfunctions, including, but not necessarily limited to, pedicle screwtesting nerve proximity monitoring, and nerve pathology monitoring.Armed with the useful information conveyed by I_(thresh) the surgeon maydetect a problem or potential problem early and then act to avoid and/ormitigate the problem.

To obtain I_(thresh) and take advantage of the useful information itprovides, the system 10 identifies and measures the peak-to-peak voltage(V_(pp)) of each EMG response corresponding to a given stimulationcurrent (I_(Stim)). Identifying the true V_(pp) of a response may becomplicated by the existence of stimulation and/or noise artifacts whichmay create an erroneous V_(pp) measurement. To overcome this challenge,the surgical system 10 of the present invention may employ any number ofsuitable artifact rejection techniques such as those shown and describedin full in the above referenced co-pending and commonly assigned PCTApp. Ser. No. PCT/US2004/025550, entitled “System and Methods forPerforming Dynamic Pedicle Integrity Assessments,” filed on Aug. 5,2004. Upon measuring V_(pp) for each EMG response, the V_(pp)information is analyzed relative to the corresponding stimulationcurrent (I_(stim)) in order to identify the minimum stimulation current(I_(Thresh)) capable of resulting in a predetermined V_(pp) EMGresponse. According to the present invention, the determination ofI_(Thresh) may be accomplished via any of a variety of suitablealgorithms or techniques.

FIGS. 17A-17D illustrates, by way of example only, the basic principlesof a threshold hunting algorithm of the present invention used toquickly find I_(thresh) during MEP monitoring. I_(thresh) is, onceagain, the minimum stimulation current (I_(stim)) that results in an EMGresponse with a V_(pp) greater than a known threshold voltage,V_(thresh). The method for finding I_(thresh) for MEP according to thepresent invention utilizes a bracketing method and a bisection method.The bracketing method quickly finds a range (bracket) of stimulationcurrents that must contain I_(thresh) and the bisection method narrowsthe bracket until I_(thresh) is known within a specified accuracy. Ifthe stimulation current threshold, I_(thresh), of a channel exceeds amaximum stimulation current, that threshold is considered out of range.

FIG. 17B illustrates the bracketing feature of the MEP threshold huntingalgorithm of the present invention. Stimulation begins at a minimumstimulation current, such as (by way of example only) 100 mA. The levelof each subsequent stimulation is doubled from the preceding stimulationlevel until a stimulation current recruits (i.e. results in an EMGresponse with a V_(pp) greater or equal to V_(thresh)). The firststimulation current to recruit (800 mA in FIG. 17B), together with thelast stimulation current to have not recruited (400 mA in FIG. 17B),forms the initial bracket.

FIGS. 17C-17D illustrate the bisection feature of the MEP thresholdhunting algorithm of the present invention. After the threshold currentI_(thresh) has been bracketed (FIG. 17B), the initial bracket issuccessively reduced via bisection to a predetermined width, such as (byway of example only) 25 mA. This is accomplished by applying a firstbisection stimulation current that bisects (i.e. forms the midpoint of)the initial bracket (600 mA in FIG. 17C). If this first bisectionstimulation current recruits, the bracket is reduced to the lower halfof the initial bracket (e.g. 400 mA and 600 mA in FIG. 17C). If thisfirst bisection stimulation current does not recruit, the bracket isreduced to the upper half of the initial bracket (e.g. 600 mA and 800 mAin FIG. 17C). This process is continued for each successive bracketuntil I_(thresh) is bracketed by stimulation currents separated by thepredetermined width (which, in this case, is 25 mA). In this exampleshown, this would be accomplished by applying a second bisectionstimulation current (forming the midpoint of the second bracket, or 500mA in this example). Because this second bisection stimulation currentis below I_(thresh), it will not recruit. As such, the second bracketwill be reduced to the upper half thereof (500 mA to 600 mA), forming athird bracket. A third bisection stimulation current forming themid-point of the third bracket (550 mA in this case) will then beapplied. Because this third bisection stimulation current is belowI_(thresh), it will not recruit. As such, the third bracket will bereduced to the upper half thereof (550 mA to 600 mA), forming a fourthbracket. A fourth bisection stimulation current forming the mid-point ofthe fourth bracket (575 mA in this case) will then be applied. Becausethe fourth bisection stimulation current is above I_(thresh), it willrecruit. The final bracket is therefore between 550 mA and 575 mA. Dueto the “response” or recruitment at 550 mA and “no response” or lack ofrecruitment at 575 mA, it can be inferred that I_(thresh) is within thisrange. In one embodiment, the midpoint of this final bracket may bedefined as I_(thresh), however, any any value falling within the finalbracket may be selected as I_(thresh) without departing from the scopeof the present invention. Depending on the active mode, the algorithmmay stop after finding I_(thresh) for the first responding channel (i.e.the channel with the lowest I_(thresh)) or the bracketing and bisectionsteps may be repeated for each channel to determine I_(thresh) for eachchannel.

For some functions, such as (by way of example) MEP monitoring, it maybe desirable to obtain I_(thresh) for each active channel each time theMEP function is performed. This is particularly advantageous whenassessing changes in I_(thresh) over time as a means to detect potentialproblems (as opposed to detecting an I_(thresh) below a predeterminedlevel determined to be safe, such as in the Screw Test modes). WhileI_(thresh) can be found for each active channel using the algorithm asdescribed above, it requires a potentially large number of stimulations,each of which is associated with a specific time delay, which can addsignificantly to the response time. Done repeatedly, it could also addsignificantly to the overall time required to complete the surgicalprocedure, which may present added risk to the patient and added costs.To overcome this drawback, a preferred embodiment of the surgical system10 boasts a multi-channel MEP threshold hunting algorithm so as toquickly determine I_(thresh) for each channel while minimizing thenumber of stimulations and thus reduce the time required to perform suchdeterminations.

The multi-channel MEP threshold hunting algorithm reduces the numberstimulations required to complete the bracketing and bisection stepswhen I_(thresh) is being found for multiple channels. The multi-channelalgorithm does so by omitting stimulations for which the result ispredictable from the data already acquired. When a stimulation signal isomitted, the algorithm proceeds as if the stimulation had taken place.However, instead of reporting an actual recruitment result, the reportedresult is inferred from previous data. This permits the algorithm toproceed to the next step immediately, without the time delay associatedwith a stimulation signal.

Regardless of what channel is being processed for I_(thresh), eachstimulation signal elicits a response from all active channels. That isto say, every channel either recruits or does not recruit in response toa stimulation signal (again, a channel is said to have recruited if astimulation signal evokes an EMG response deemed to be significant onthat channel, such as V_(pp) of approximately 100 uV). These recruitmentresults are recorded and saved for each channel. Later, when a differentchannel is processed for I_(thresh), the saved data can be accessed and,based on that data, the algorithm may omit a stimulation signal andinfer whether or not the channel would recruit at the given stimulationcurrent.

There are two reasons the algorithm may omit a stimulation signal andreport previous recruitment results. A stimulation signal may be omittedif the selected stimulation current would be a repeat of a previousstimulation. By way of example only, if a stimulation current of 100 mAwas applied to determine I_(thresh) for one channel, and a stimulationat 100 mA is later required to determine I_(thresh) for another channel,the algorithm may omit the stimulation and report the previous results.If the specific stimulation current required has not previously beenused, a stimulation signal may still be omitted if the results arealready clear from the previous data. By way of example only, if astimulation current of 200 mA was applied to determine I_(thresh) for aprevious channel and the present channel did not recruit, when astimulation at 100 mA is later required to determine I_(thresh) for thepresent channel, the algorithm may infer from the previous stimulationthat the present channel will not recruit at 100 mA because it did notrecruit at 200 mA. The algorithm may therefore omit the stimulation andreport the previous result.

FIG. 18 illustrates (in flowchart form) a method by which themulti-channel MEP threshold hunting algorithm determines whether tostimulate, or not stimulate and simply report previous results. Thealgorithm first determines if the selected stimulation current hasalready been used (step 202). If the stimulation current has been used,the stimulation is omitted and the results of the previous stimulationare reported for the present channel (step 204). If the stimulationcurrent has not been used, the algorithm determines I_(recruit) (step206) and I_(norecruit) (step 208) for the present channel. I_(recruit)is the lowest stimulation current that has recruited on the presentchannel. I_(norecruit) is the highest stimulation current that hasfailed to recruit on the present channel. The algorithm next determineswhether I_(recruit) is greater than I_(norecruit) (step 210). AnI_(recruit) that is not greater than I_(norecruit) is an indication thatchanges have occurred to I_(thresh) on that channel. Thus, previousresults may not be reflective of the present threshold state and thealgorithm will not use them to infer the response to a given stimulationcurrent. The algorithm will stimulate at the selected current and reportthe results for the present channel (step 212). If I_(recruit) isgreater than I_(norecruit), the algorithm determines whether theselected stimulation current is higher than I_(recruit), lower thanI_(norecruit), or between I_(recruit) and I_(norecruit) (step 214). Ifthe selected stimulation current is higher than I_(recruit), thealgorithm omits the stimulation and reports that the present channelrecruits at the specified current (step 216). If the selectedstimulation current is lower than I_(norecruit), the algorithm infersthat the present channel will not recruit at the selected current andreports that result (step 218). If the selected stimulation currentfalls between I_(recruit) and I_(norecruit), the result of thestimulation cannot be inferred and the algorithm stimulates at theselected current and reports the results for the present channel (step212). This method may be repeated until I_(thresh) has been determinedfor every active channel.

In the interest of clarity, FIGS. 19A-19C demonstrate use of themulti-channel MEP threshold hunting algorithm to determine I_(thresh) ononly two channels. It should be appreciated, however, that themulti-channel algorithm is not limited to finding I_(thresh) for twochannels, but rather it may be used to find I_(thresh) for any number ofchannels, such as (for example) eight channels according to a preferredembodiment of the surgical system 10. With reference to FIG. 19A,channel 1 has an I_(thresh) to be found of 625 mA and channel 2 has anI_(thresh) to be found of 425 mA. I_(thresh) for channel 1 is foundfirst, using the bracketing and bisection methods discussed above, asillustrated in FIG. 17B. Bracketing begins at the minimum stimulationcurrent (for the purposes of example only) of 100 mA. As this is thefirst channel processed and no previous recruitment results exist, nostimulations are omitted. The stimulation current is doubled with eachsuccessive stimulation until a significant EMG response is evoked at 800mA. The initial bracket of 400 mA-800 mA is bisected, using thebisection method described above, until the stimulation threshold,I_(thresh), is contained within a final bracket separated by theselected width or resolution (again 25 mA). In this example, the finalbracket is 600 mA-625 mA. I_(thresh) may be defined as any point withinthe final bracket or as the midpoint of the final bracket (612.5 mA inthis case). In either event, I_(thresh) is selected and reported asI_(thresh) for channel 1.

Once I_(thresh) is found for channel 1, the algorithm turns to channel2, as illustrated in FIG. 19C. The algorithm begins to process channel 2by determining the initial bracket, which is again 400 mA-800 mA. Allthe stimulation currents required in the bracketing state were used indetermining I_(thresh) for channel 1. The algorithm refers back to thesaved data to determine how channel 1 responded to the previousstimulations. From the saved data, the algorithm may infer that channel2 will not recruit at stimulation currents of 100, 200, and 400 mA, andwill recruit at 800 mA. These stimulations are omitted and the inferredresults are displayed. The first bisection stimulation current selectedin the bisection process (600 mA in this case), was previously used and,as such, the algorithm may omit the stimulation and report that channel2 recruits at that stimulation current. The next bisection stimulationcurrent selected (500 mA in this case) has not been previously used and,as such, the algorithm must determine whether the result of astimulation at 500 mA may still be inferred. In the example shown,I_(recruit) and I_(norecruit) are determined to be 600 mA and 400 mA,respectively. Because 500 mA falls in between I_(recruit) andI_(norecruit), the algorithm may not infer the result from the previousdata and, as such, the stimulation may not be omitted. The algorithmthen stimulates at 500 mA and reports that the channel recruits. Thebracket is reduced to the lower half (making 450 mA the next bisectionstimulation current). A stimulation current of 450 mA has not previouslybeen used and, as such, the algorithm again determines I_(recruit) andI_(norecruit) (500 mA and 400 mA in this case). The selected stimulationcurrent (450 mA) falls in between I_(recruit) an I_(norecruit) and, assuch, the algorithm stimulates at 450 mA and reports the results. Thebracket now stands at its final width of 25 mA (for the purposes ofexample only). I_(thresh) may be defined as any point within the finalbracket or as the midpoint of the final bracket (412.5 mA in this case).In either event, I_(thresh) is selected and reported as I_(thresh) forchannel 2.

Although the multi-channel MEP threshold hunting algorithm is describedabove processing channels in numerical order, it will be understood thatthe actual order in which channels are processed is immaterial. Thechannel processing order may be biased to yield the highest or lowestthreshold first (discussed below) or an arbitrary processing order maybe used. Furthermore, it will be understood that it is not necessary tocomplete the algorithm for one channel before beginning to process thenext channel, provided that the intermediate state of the algorithm isretained for each channel. Channels are still processed one at a time.However, the algorithm may cycle between one or more channels,processing as few as one stimulation current for that channel beforemoving on to the next channel. By way of example only, the algorithm maystimulate at 100 mA while processing a first channel for I_(thresh).Before stimulating at 200 mA (the next stimulation current in thebracketing phase), the algorithm may cycle to any other channel andprocess it for the 100 mA stimulation current (omitting the stimulationif applicable). Any or all of the channels may be processed this waybefore returning to the first channel to apply the next stimulation.Likewise, the algorithm need not return to the first channel tostimulate at 200 mA, but instead may select a different channel toprocess first at the 200 mA level. In this manner, the algorithm mayadvance all channels essentially together and bias the order to find thelower threshold channels first or the higher threshold channels first.By way of example only, the algorithm may stimulate at one current leveland process each channel in turn at that level before advancing to thenext stimulation current level. The algorithm may continue in thispattern until the channel with the lowest I_(thresh) is bracketed. Thealgorithm may then process that channel exclusively until I_(thresh) isdetermined, and then return to processing the other channels onestimulation current level at a time until the channel with the nextlowest I_(thresh) is bracketed. This process may be repeated untilI_(thresh) is determined for each channel in order of lowest to highestI_(thresh). If I_(thresh) for more than one channel falls within thesame bracket, the bracket may be bisected, processing each channelwithin that bracket in turn until it becomes clear which one has thelowest I_(thresh). If it becomes more advantageous to determine thehighest I_(thresh) first, the algorithm may continue in the bracketingstate until the bracket is found for every channel and then bisect eachchannel in descending order.

FIG. 20 illustrates a further feature of the MEP threshold huntingalgorithm of the present invention, which advantageously provides theability to further reduce the number of stimulations required to findI_(thresh) when an I_(thresh) value has previously been determined for aspecific channel. In the event that a previous I_(thresh) determinationexists for a specific channel, the algorithm may begin by merelyconfirming the previous I_(thresh) rather than beginning anew with thebracketing and bisection methods. The algorithm first determines whetherit is conducting the initial threshold determination for the channel orwhether there is a previous I_(thresh) determination (step 220). If itis not the initial determination, the algorithm confirms the previousdetermination (step 222) as described below. If the previous thresholdis confirmed, the algorithm reports that value as the present I_(thresh)(step 224). If it is the initial I_(thresh) determination, or if theprevious threshold cannot be confirmed, then the algorithm performs thebracketing function (step 226) and bisection function (step 228) todetermine I_(thresh) and then reports the value (step 224).

FIG. 21 illustrates, by way of example only, a method employed by theMEP threshold hunting algorithm for confirming a previous threshold. Theconfirmation step attempts to ascertain whether I_(thresh) has movedfrom its last known value. To do this, the algorithm applies twostimulation currents, one at or just above the threshold value and theother just below the threshold value. If the stimulation at or aboveI_(thresh) recruits and the stimulation just below I_(thresh) does notrecruit, then the threshold has not moved and the algorithm may reportthat value as I_(thresh) and proceed to process another channel. If thestimulation just below I_(thresh) recruits, it may be concluded thatI_(thresh) has decreased and likewise if the stimulation at or justabove I_(thresh) fails to recruit, it may be concluded that I_(thresh)has increased.

If I_(thresh) cannot be confirmed, the algorithm enters the bracketingstate. Rather than beginning the bracketing state from the minimumstimulation current, however, the bracketing state may begin from theprevious I_(thresh). The bracketing may advance up or down depending onwhether I_(thresh) has increased or decreased. By way of example only,if the previous value of I_(thresh) was 400 mA, the confirmation stepmay stimulate at 400 mA and 375 mA. If the stimulation at 400 mA failsto evoke a significant response, it may be concluded that the I_(thresh)has increased and the algorithm will bracket up from 400 mA. When thealgorithm enters the bracketing state, the increment used in theconfirmation step (ie. 25 mA in this example) is doubled. Thus, in thisexample, the algorithm stimulates at 450 mA. If the channel fails torecruit at this current level, the increment is doubled again (100 mA inthis example) and the algorithm stimulates at 550 mA. This process isrepeated until the maximum stimulation current is reached or the channelrecruits, at which time the bisection function may be performed. If,during the confirmation step, the stimulation current just below thepreviously determined I_(thresh) recruits, it may be concluded thatI_(thresh) for that channel has decreased and the algorithm may bracketdown from that value (375 mA in this case). Thus, in this example, thealgorithm would double the increment to 50 mA and stimulate at 325 mA.If the channel still recruits at this stimulation current, the incrementis doubled again to 100 mA such that the algorithm stimulates at 225 mA.This process is repeated until the minimum stimulation current isreached or the channel fails to recruit, at which time the algorithm mayperform the bisection function. When determining I_(thresh) for multiplechannels with previously determined I_(thresh) values, this techniquemay be performed for each channel, in turn, in any order. Againstimulations may be omitted and the algorithm may begin processing a newchannel before completing the algorithm for another channel, asdescribed above.

Although the hunting algorithm is discussed herein in terms of findingI_(thresh) (the lowest stimulation current that evokes a predeterminedEMG response), it is contemplated that alternative stimulationthresholds may be useful in assessing the health of the spinal cord ornerve monitoring functions and may be determined by the huntingalgorithm. By way of example only, the hunting algorithm may be employedby the system 10 to determine a stimulation voltage threshold,Vstim_(thresh). This is the lowest stimulation voltage (as opposed tothe lowest stimulation current) necessary to evoke a significant EMGresponse, V_(thresh). Bracketing, bisection and monitoring states areconducted as described above for each active channel, with bracketsbased on voltage being substituted for the current based bracketspreviously described. Moreover, although described above within thecontext of MEP monitoring, it will be appreciated that the algorithmsdescribed herein may also be used for determining the stimulationthreshold (current or voltage) for any other EMG related functions,including but not limited to bone integrity (e.g. pedicle screw test),nerve detection, and nerve root retraction.

According to one embodiment of the present invention, the surgicalsystem 10 may perform the MEP function in either of two modes: Automaticmode and Manual mode. In one embodiment, these MEP modes are selectablefrom the drop-down function menu 104 of FIG. 4. In Automatic mode, themulti-channel MEP threshold hunting algorithm described above isutilized to determine a baseline I_(thresh) for each channel, preferablyprior to or in the early stages of a surgical procedure. It should beappreciated, however, that a new baseline I_(thresh) may be determinedat any time during the procedure at the option of the surgeon or otherqualified operator. Having determined a baseline I_(thresh) for eachchannel, subsequent monitoring may be performed as desired throughoutthe procedure and recovery period to obtain updated I_(thresh) valuesfor each channel. Each new determination of I_(thresh) is compared bythe surgical system 10 to the baseline I_(thresh) for the appropriatechannel. The difference (ΔI_(thresh) between the baseline I_(thresh) andthe new I_(thresh) is calculated by the system 10 and the ΔI_(thresh)value is compared to predetermined “safe” and “unsafe” values. IfΔI_(thresh) is greater than the predetermined safe level, the user isalerted to a potential complication and action may be taken to avoid ormitigate the problem. The speed with which the multi-channel MEPthreshold hunting algorithm is able to determine I_(thresh) across allchannels, and the simplicity with which the data communicated to theuser may be interpreted, allows the user to increase the frequency ofMEP monitoring conducted during a procedure without a concurrentincrease in overall surgery time. This provides significant benefit tothe patient by reducing the time intervals in between MEP monitoringepisodes during which an injury to the spinal cord may go undetected.

In use, various features and parameters of the MEP function may becontrolled and/or adjusted by the operator. In one example such controlmay be exercised from an “MEP Auto” mode setup screen, shown by way ofexample only in FIG. 22. Using this screen, the operator may turndifferent EMG channels on or off, set the date and time, conduct animpedance test to check the electrical connection between the EMGelectrodes and the patient's skin, shutdown the system 10, and changeone or more stimulation settings. Up and down control arrows 126 may beselected to increase or decrease the maximum allowable stimulationcurrent (I), the pulse width (W), the number of pulses per stimulationsignal (N), and the interpulse gap (G). MEP responses may be affected bya number of variables and the current level needed to evoke asignificant response (i.e. I_(thresh)) may vary based on the parametersof the stimulation signal used. By varying the stimulation signalparameters, the operator may optimize the stimulation signal for eachpatient (e.g. find the signal parameters that result in the lowestI_(thresh) values). In one embodiment, signal optimization may also becarried out automatically by the surgical system 10. Automatic signaloptimization may be initiated by selecting a “signal optimization” tab128, which will cause the surgical system 10 to perform a series ofstimulations varying the different signal parameters until a signalconfiguration resulting in the lowest I_(thresh) values is determined.If more than one pair of stimulation electrodes 23 have been arranged onthe head, then signal optimization may be advantageous in determiningwhich electrodes 23 generate a better response. It may also beadvantageous to optimize the signal after any instance where the signalis not detected or otherwise deteriorates.

The polarity of the stimulation signal also has an effect on the MEPresponse. Positive phase stimulations may, for example, result in alower I_(thresh) values for muscles on the left side of the body thanthe right side of the body (or vice versa). To achieve the best MEPresponse values for all channels, it may be desirable to includestimulation signals of both positive and negative polarity. A polarityauto-switching feature may be controlled using on/off tabs 130. Whenpolarity auto-switching is on, stimulation signals from MEP stimulator21 alternate between a positive phase and a negative phase. The left andright sides of the brain may respond differently to positive andnegative pulses. Each stimulation signal is used twice, once as apositive phase signal and once as a negative phase signal, before thehunting algorithm advances to the next stimulation current level. By wayof example, if the algorithm begins stimulations at a minimumstimulation current of 100 mA, a first stimulation signal will include 1to 8 positive phase pulses of 100 mA. A second stimulation signal willthen include the same number of negative phase pulses of the samecurrent, 100 mA. After stimulation results have been determined for thefirst current level using both polarities, the algorithm will stimulateat the next current level, in this case 200 mA, first with a positivephase signal followed by the negative phase signal. The algorithm willcontinue in this pattern until I_(thresh) is determined for eachchannel. The order in which positive and negative phases are used is notimportant and may be reversed such that the first stimulation signalincludes negative phase pulses and the second signal follows withpositive phase pulses. When polarity auto-switching is turned off, theoperator may select the polarity to be used from the MEP display screen(FIG. 23) or the stimulations may occur according to a default setting,which may be set as either positive or negative polarity.

The surgical system 10 includes the ability to remind the user toperform an MEP stimulation, which can be controlled from the setupscreen. Using up and down control arrows 126, the operator may setand/or change a time interval for receiving stimulation reminders. Aftereach MEP monitoring episode, the system 10 will initiate a timercorresponding to the selected time interval and, when the time haselapsed, a stimulation reminder will be activated. The stimulationreminder may include, by way of example only, any one of, or combinationof, an audible tone, voice recording, screen flash, pop up window,scrolling message, or any other such alert to remind the operator totest MEP again.

FIGS. 23-24 depict exemplary screen displays for automatic mode of theMEP function, one with alpha-numeric information only (FIG. 23) and onewith alpha-numeric information along with optional EMG waveforms 116(FIG. 24). A mode indicator tab 110 indicates that “MEP Automatic” isthe selected mode. Polarity of the stimulation signal may be manuallyselected using the plus (+) and minus (−) polarity tabs 152. MEPstimulation may be initiated by selecting the stimulation start button142 labeled (by way of example only) “MEP Stim.” A stimulation bar 112graphically depicts the stimulation current level. A channel window 144is included for each EMG channel. The channel window 144 may displayinformation including the channel number, myotome name, and associatedspinal levels 164. Each channel window 144 may also display the baselinethreshold, the most recent detected threshold value, and the differencebetween the two values. In the event the system 10 detects a significantdifference (ΔI_(thresh)) between the baseline threshold and the mostrecent threshold on a particular channel, the associated channel window144 may preferably be highlighted with a predetermined color (e.g. red)to indicate the potential danger to the surgeon. Preferably, thestimulation results are displayed to the surgeon along with a color codeso that the operator may easily comprehend the situation and avoidneurological impairment to the patient (e.g. red for “danger,” yellowfor “caution” and green for “safe”). In the example shown, the red isdenoted with reference numeral 166, yellow is denoted with referencenumeral 168, and green is denoted with reference numeral 170. In oneembodiment of the MEP mode, set forth by way of example only, a greenchannel window 144 corresponds to a stimulation threshold change,ΔI_(thresh), of less than 25 mA, a yellow window denotes a stimulationthreshold change of between 25-150 mA, and a red channel window 144denotes a stimulation threshold change of greater than 150 mA.Annotation buttons 146 allow the surgeon to quickly annotate a thresholdresponse with useful information that may not be automaticallydetectable by the system 10.

EMG sensitivity controls 148 and a Free-Run status control 150 are alsoprovided on the screen. A check mark displayed in the free-run statuscontrol 150 indicates that free-run EMG mode is activated. When free-runis activated, the surgical system 10 continuously monitors EMGelectrodes 18 for spontaneous nerve activity unless another mode, suchas MEP, is active. Upon completion of an MEP episode, the surgicalsystem 10 may automatically transition into free-run EMG monitoring, inwhich actual EMG waveforms are continuously displayed in real-time. Indoing so, the user may be alerted to any nerve activity occurringunexpectedly. An audio pick-up (not shown) may also be provided as anoptional feature according to the present invention. In some cases, whena nerve is stretched or compressed, it will emit a burst or train ofspontaneous nerve activity. The audio pick-up is capable of transmittingsounds representative of such activity such that the surgeon can monitorthis response on audio to help him or her determine if there has beenstress to the nerve.

In manual MEP mode, the user simply selects a stimulation current andthe system 10 determines whether or not the selected current evokes apredetermined EMG response. The user may be alerted to a potentialcomplication if no response is detected from an EMG channel that hadpreviously responded to a stimulation signal of the same or lesseramplitude. In one embodiment, set forth by way of example only, the usermay determine for each channel a baseline stimulation current at whichthe stimulation signal evokes a response. Thereafter, the user maystimulate at each baseline and determine whether the correspondingchannel still responds. Alternatively, a supramaximal current may bedetermined at which all channels show a response. Subsequentstimulations signals may be delivered at the same supramaximal currentlevel and should continue to evoke a response. Subsequent absence of aresponse may be indicative of a problem with the spinal cord.

FIG. 25 shows, by way of example only, an exemplary setup screen for theMEP manual mode. In similar fashion to the setup screen previouslydescribed for “MEP Automatic” mode, the operator may turn different EMGchannels on or off, set the date and time, conduct an impedance test tocheck the electrical connection between the EMG electrodes and thepatient's skin, shutdown the system 10, and change one or morestimulation settings. Up and down control arrows 126 may be selected toincrease or decrease the maximum allowable stimulation current (I), thepulse width (W), the number of pulses per stimulation signal (N), theinterpulse gap (G), and a stimulation reminder time interval. Byselecting the “signal optimization” tab 128, as described above, thesystem 10 will initiate a series of stimulation signals and determine anoptimum configuration of stimulation signal parameters to obtain thebest MEP responses.

FIGS. 26-27 illustrate exemplary screen displays of “MEP Manual” mode,one with alpha-numeric information only (FIG. 26) and one withalpha-numeric information along with optional EMG waveforms 116 (FIG.27). A mode indicator tab 110 indicates that “MEP Manual” mode isselected. An amplitude dial 153 is used to manually set the stimulationcurrent. The amplitude setting may be increased or decreased inincrements of 25 mA using the amplitude selection buttons 154 labeled(by way of example only) “+25” and “−25”. More precise amplitudeselections may be made by sweeping the dial indicator 156 around thedial face 158. The exact dial setting 160 is indicated in the center ofthe dial 153. Polarity controls 152 may be used to set the desiredpolarity of the stimulation signal. MEP stimulation may be initiated atthe selected current amplitude shown in the dial setting 160 by pressingthe stimulation start button 142 labeled (by way of example only) “MEPStim.” The stimulation bar 112 graphically depicts the stimulationcurrent level. Each EMG channel includes a channel window 144. Providedin the channel window 144 is information including the channel number,myotome name, and associated spinal levels 164. Stimulation results aredisplayed in the form of “Yes” or “No” responses (or equivalent indicia,such as a “check” mark for yes and an “X” for no) indicated in theappropriate channel windows 144. If baselines were determined, thebaseline values may also be shown in the appropriate channel windows144. Channel windows 144 indicating a “No” response are preferablycolored red (denoted 166) to clearly indicate to the operator the lackof response suggesting a potential complication. Channel windowsindicating a “Yes” response are preferably colored green (denoted 170)to clearly indicate to the operator the presence of a responsesuggesting no potential problems. Annotation buttons 146 allow thesurgeon to quickly annotate a stimulation response with usefulinformation that may not be automatically detectable by the system 10.EMG sensitivity controls 148 and a Free-Run status control 150 are alsoprovided on the screen.

In addition to the MEP techniques described above, or instead of thesetechniques, the surgical system 10 is capable of monitoring spinal cordhealth via any number of different manners using additional data fromthe MEP test. For example, the system 10 may monitor changes inamplitude of the EMG responses as an indicator of spinal cord health.The system 10 may detect changes over time of the peak-to-peak voltage(i.e. amplitude of the EMG response) relative to a given stimulationsignal current (shown in FIG. 28), which may be indicative of damage inspinal cord inhibiting signal transmission. The system 10 may measurethe V_(pp) response corresponding to a stimulation using the constantcurrent, I_(stim). The system 10 may then compare the V_(pp) of thepresent reading to a proceeding or baseline reading taken in response toa stimulation signal of the constant I_(stim). The difference betweenthe present V_(pp) value and the prior V_(pp) value may then becommunicated via the onscreen display 26. In one embodiment, “MEPAutomatic” mode may be used initially to quickly determine a suitableI_(stim) current. This may be accomplished by determining I_(thresh) foreach active channel using the multi-channel MEP threshold huntingalgorithm. I_(stim) may then preferably be selected with an amplitudeslightly greater than the largest I_(thresh). The surgical system 10 mayalso monitor spinal cord health by assessing the timing of the MEPresponse waveform. There is a characteristic delay from the time astimulation pulse is delivered to the motor cortex to the time acorresponding muscle response is detected. This delay or latency periodmay be detected by the system 10. An increase in the latency period oversuccessive measurements, as depicted in FIG. 29, may be a furtherindicator of problems with the stimulation signal transmission throughthe spinal cord. As such, the system 10 may detect an increase inlatency with respect to a previously recorded baseline reading or thepreceding reading, and the information may be conveyed to the operatorvia the display 26. The surgical system 10 may also monitor spinal cordhealth by assessing the morphology of an MEP response waveform. MEPresponse waveforms (EMG) are often complex polyphasic waves with severalpeaks. A baseline stimulation signal amplitude may be determined atwhich a polyphasic response is consistently produced. Subsequent changesto a bi-phasic or tri-phasic waveform, as illustrated in FIG. 30, inresponse to a stimulation signal of the same current may again be anindication of problems affecting the health of the spinal cord.

In a second broad aspect of the present invention, the surgical system10 is capable of assessing the health of the spinal cord via SSEPmonitoring. The system performs SSEP monitoring by stimulatingperipheral sensory nerves that exit the spinal cord below the level ofsurgery and measuring the electrical action potential from electrodeslocated on the nervous system tract superior to the surgical targetsite. The neural electrical signal is then analyzed in relation to thestimulation pulse, resulting in quantitative information related to thehealth of the spinal cord, which is then conveyed to the surgeon via thedisplay 26. SSEP stimulation may be conducted on the Posterior Tibialnerve with the electrical signal being recorded from any suitablelocation, including but not limited to the skin overlying the secondcervical (C2) vertebrae or the skin of the scalp. Accordingly, a pair ofperipheral nerve stimulation (PNS) electrodes 25 may be positioned onthe skin above the Posterior Tibial nerve, located at the ankle, andrecording electrodes 41 may be placed on the skin above the C2 vertebra.The stimulation pulse is delivered by the patient module 14, which iscoupled to the PNS electrodes 25 via an accessory cable 30. AlthoughSSEP stimulation and recording is discussed with respect to thePosterior Tibial nerve and C2 vertebra, it will be appreciated that SSEPstimulation may applied to any number of peripheral sensory nerves,including but not necessarily limited to the Ulnar and Median nerves inthe wrist, as well as directly stimulating the spinal cord inferior tothe expected level of potential damage. Likewise, it will be appreciatedthat the recording site may be located anywhere along the nervous systemsuperior to the spinal level at risk during the procedure, including butnot necessarily limited to any suitable location on the scalp.

When the peripheral sensory nerve is stimulated an electrical pulseascends from the nerve to the spinal cord and up into the brain. Damagein the spinal cord can disrupt transmission of the signal up the cord,resulting in a weakened or delayed signal at the recording site. Thesurgical system 10 detects such disruptions by measuring the amplitudeof the stimulation signal waveform when it reaches the recording site,as well as the latency period (time signal takes to travel from thestimulation site to the recording site). The system 10 comparesamplitude measurements to a previously recorded baseline amplitude orthe preceding measurement, and the difference between them is viewed onthe display 26. Similarly, latency measurements are compared to apreviously recorded baseline latency or the preceding measurement andthe difference value is shown on the display 26. A decrease in amplitudeor an increase in latency may alert the surgeon to damage in the spinalcord and corrective measures may be taken to avoid or mitigate suchdamage.

FIG. 31 is an exemplary illustration of an onscreen display for the SSEPmode. A mode indicator tab 110 indicates that SSEP is the selected mode.An amplitude dial 153 is used to set the stimulation current. Theamplitude setting may be increased or decreased in increments of 5 mAusing the amplitude selection buttons 154 labeled (by way of exampleonly) “+5” and “−5”. More precise amplitude selections may be made bysweeping the dial indicator 156 around the dial face 158. The precisedial setting 160 is indicated in the center of the dial 153. Polaritycontrols 152 may be used to set the desired polarity of the stimulationsignal. SSEP stimulation may be initiated at the selected currentamplitude shown in the dial setting 160 by pressing the SSEP stimulationstart button 143 labeled (by way of example only) “SSEP Stim.” Astimulation bar 112 graphically depicts the stimulation current level.By way of example only, the SSEP information communicated to the user inthe window 170 may relate to the changes in latency and amplitudebetween baseline values and the present reading or test. In the exampleshown, the latency information includes baseline (15 μs), presentreading (19 μs), and difference (4 μs), while the amplitude informationincludes baseline (12 μv), present reading (11 μv), difference (1 μv),and the percentage change (8%). This information may be interpreted asbeing indicative of a problem with the neural pathways being tested. Forexample, a reduction in amplitude of 50% between baseline and a presentreading is indicative of a potential problem with the neural pathwaybeing tested. The numerical results may be accompanied by the same colorcode discussed above. Red is used when the decrease in amplitude orincrease in latency is within a predetermined unsafe level. Greenindicates that the measured increase or decrease is within apredetermined safe level. Yellow is used for measurements that arebetween the predetermined unsafe and safe levels. FIG. 32 depicts, byway of example only, a setup screen for SSEP. Using this screen, theoperator may change the stimulation current level using up and downcontrol arrows 126, set the date and time, conduct an impedance test tocheck the electrical connection between the recording electrodes and thepatients skin, and shutdown the system 10.

In a third significant aspect of the present invention, the surgicalsystem 10 may conduct other nerve monitoring functions, including butnot necessarily limited to, neuromuscular pathway assessments to ensurethat muscle relaxants, paralytic agents, and/or anesthetics are nolonger affecting the neuromuscular pathway (Twitch Test), bone integritytesting (e.g. Pedicle Screw Test), nerve proximity testing (Detection),and nerve pathology monitoring (Nerve Root Retraction). These additionalfunctions have been described in detail in the NeuroVision Applicationsreferenced above, the entire contents of which are expresslyincorporated by reference as if set forth herein in their entirety, andwill thus be described here only briefly. In similar fashion to the MEPfunction discussed above, the system 10 conducts nerve monitoringfunctions by electrically stimulating a nerve via one or morestimulation electrodes positioned on the surgical accessories 32,monitoring the corresponding muscle response of muscles innervated bythe nerve, and analyzing the muscle response in relation to thestimulation signal to determine one of neuromuscular pathway function,bone integrity, nerve proximity, and nerve pathology. In a preferredembodiment, EMG monitoring may be conducted on the same muscle groupsmonitored for the MEP function, as illustrated above in Tables 2, 3, 4.In this manner, the EMG electrodes 18 need be placed only one time,prior to or at the beginning of the surgery, and may be used to monitorEMG responses for all the various functions of the system 10.

The surgical system 10 performs neuromuscular pathway (NMP) assessments(Twitch Test) by electrically stimulating a peripheral nerve via PNSelectrodes 25 placed on the skin over the nerve or by direct stimulationof a spinal nerve using a surgical accessory such as balled-tipped testprobe 36. Evoked responses from the muscles innervated by the stimulatednerve are detected and recorded, the results of which are analyzed and arelationship between at least two responses or a stimulation signal anda response is identified. The identified relationship provides anindication of the current state of the NMP. The identified relationshipmay include, but is not necessarily limited to, one or more of magnituderatios between multiple evoked responses and the presence or absence ofan evoked response relative to a given stimulation signal or signals.Details of the test indicating the state of the NMP and the relativesafety of continuing on with nerve testing are conveyed to the surgeonvia the screen display 26.

The surgical system 10 may test the integrity of pedicle holes (duringand/or after formation) and/or screws (during and/or afterintroduction). The screw test probe 36 is placed in the screw hole priorto screw insertion or placed on the installed screw head and astimulation signal is applied. The insulating character of bone willprevent the stimulation current, up to a certain amplitude, fromcommunicating with the nerve, thus resulting in a relatively highI_(thresh), as determined via the basic threshold hunting algorithmdescribed above. However, in the event the pedicle wall has beenbreached by the screw or tap, the current density in the breach areawill increase to the point that the stimulation current will passthrough to the adjacent nerve roots and they will depolarize at a lowerstimulation current, thus I_(thresh) will be relatively low. In analternative embodiment, screw test probe 36 may be replaced with anelectric coupling device 42, 52 which may be utilized to couple asurgical tool, such as for example, a tap member 72 or a bone awl 74, tothe surgical system 10. In this manner, a stimulation signal may bepassed through the surgical tool and screw testing can be performedwhile the tool is in use. Thus, screw testing may be performed duringpilot hole formation by coupling the bone awl 74 to the surgical system10 and during pilot hole preparation by coupling the tap 72 to thesystem 10. Likewise, by coupling a pedicle screw to the surgical system10 (such as via pedicle screw instrumentation), screw testing may beperformed during screw introduction.

The surgical system 10 may perform nerve proximity testing (Detection)to ensure safe and reproducible access to surgical target sites. Usingthe surgical access components 62-66, the system 10 detects theexistence of neural structures before, during, and after theestablishment of an operative corridor through (or near) any of avariety of tissues having such neural structures which, if contacted orimpinged, may otherwise result in neural impairment for the patient. Thesurgical access components 62-66 are designed to bluntly dissect thetissue between the patient's skin and the surgical target site. Cannulaeor dilators of increasing diameter, which are equipped with one or morestimulating electrodes, are advanced towards the target site until asufficient operating corridor is established. As the cannulae ordilators are advanced to the target site electrical stimulation signalsare emitted via the stimulation electrodes. The stimulation signal willstimulate nerves in close proximity to the stimulation electrode and thecorresponding EMG response is monitored. As a nerve gets closer to thestimulation electrode, the stimulation current (I_(stim)) required toevoke a muscle response decreases. I_(thresh) is calculated, using thebasic threshold hunting algorithm described above, providing a measureof the communication between the stimulation signal and the nerve andthus giving a relative indication of the proximity between accesscomponents and nerves.

Additional and/or alternative surgical access components such as, by wayof example only, a tissue retraction assembly 70 (FIG. 1) may be coupledto the system 10 and employed to provide safe and reproducible access toa surgical target site. Tissue retraction assembly 70 and variousembodiments and uses thereof have been shown and described in the abovereferenced co-pending and commonly assigned U.S. patent application Ser.No. 10/967,668, entitled “Surgical Access System and Related Methods,”filed on Oct. 18, 2004, the entire contents of which are expresslyincorporated by reference as if set forth herein in their entirety.

The surgical system 10 preferably accomplishes neural pathologymonitoring via the Nerve Retractor function, specifically by determininga baseline stimulation threshold with direct contact between the nerveretractor 76 and the nerve, prior to retraction. Subsequent stimulationthresholds are determined during retraction and they are compared to thebaseline threshold. Significant changes in the stimulation threshold mayindicate potential trauma to the nerve caused by the retraction and aredisplayed to the user on the display 26. An increase in I_(thresh) overtime is an indication that the nerve function is deteriorating andretraction should be reduced or stopped altogether to prevent permanentdamage.

With reference to FIG. 33, the stimulation results, includingannotations, may be compiled in a surgical report chronicling all nervemonitoring functions conducted during the procedure. In one embodimentthe report may be printed immediately from one or more printers locatedin the operating room or copied to any of a variety of memory devicesknown in the prior art, such as, by way of example only, a floppy disk,or USB memory device. The system 10 may generate either a full report ora summary report depending the particular needs of the user, who mayselect one or both using the GUI screen display 26, as illustrated inFIG. 34. FIGS. 35A-35C are an exemplary representation of a summaryreport generated by system 10. The summary report includes space forpatient, physician, and procedural information and surgeon operativenotes along with the stimulation results. The stimulation results,including any annotated data, are preferably displayed in chronologicalorder for each function. FIGS. 36A-36E are an exemplary representationof a full report generated by the system 10. The full report alsoincludes space for patient and physician information and surgeonoperative notes. The full stimulation results are displayed inchronological order regardless of the particular function.

The control unit 12 is configured to monitor the system statusthroughout its use. In the event the control unit 12 detects anaberration an error log is created in which the details of the error aredescribed and stored to assist in later troubleshooting and systemcorrection. To service the system 10, the error logs may be accesseddirectly from the control unit 12 hardware and software. In addition,error logs may be downloaded onto any of a number of suitable media tofacilitate data transfer between remote locations. By way of exampleonly, the error logs may be downloaded to a USB memory device, floppydisk, CD, or DVD. By way of further example, the error logs may bedownloaded onto a network and transmitted to remote locations via theInternet or other data transfer systems.

It will be readily appreciated that various modifications may beundertaken, or certain steps or algorithms omitted or substituted,without departing from the scope of the present invention. By way ofexample only, several alternative methods will now be described. Ratherthan identifying the stimulation current threshold (I_(Thresh)) based ona predetermined V_(Thresh) it is also within the scope of the presentinvention to determine I_(Thresh) via linear regression. This may beaccomplished via, by way of example only, the linear regressiontechnique disclosed in commonly owned and co-pending U.S. patentapplication Ser. No. 09/877,713, filed Jun. 8, 200 and entitled“Relative Nerve Movement and Status Detection System and Methods,” theentire contents of which is hereby expressly incorporated by referenceas if set forth in this disclosure in its entirety.

Additionally, the nerve pathology monitoring function described abovemay be employed for the purpose of monitoring the change, if any, inperipheral nerves during the course of the procedure. This may beaccomplished by positioning additional stimulation electrodes anywhereon a surgical accessory that is likely to come in contact with aperipheral nerve during a surgical procedure. Recruitment curves orother data can be generated and assessed in the same fashion describedabove.

Moreover, although described with reference to the surgical system 10,it will be appreciated as within the scope of the invention to performMEP and SSEP monitoring as described herein with any number of differentneurophysiology based testing, including but not limited to the “NIMSPINE” testing system offered by Medtronic Sofamor Danek, Inc.

While this invention has been described in terms of a best mode forachieving this invention's objectives, it will be appreciated by thoseskilled in the art that variations may be accomplished in view of theseteachings without deviating from the spirit or scope of the presentinvention. For example, the present invention may be implemented usingany combination of computer programming software, firmware or hardware.As a preparatory step to practicing the invention or constructing anapparatus according to the invention, the computer programming code(whether software or firmware) according to the invention will typicallybe stored in one or more machine readable storage mediums such as fixed(hard) drives, diskettes, optical disks, magnetic tape, semiconductormemories such as ROMs, PROMs, etc., thereby making an article ofmanufacture in accordance with the invention. The article of manufacturecontaining the computer programming code is used by either executing thecode directly from the storage device, by copying the code from thestorage device into another storage device such as a hard disk, RAM,etc. or by transmitting the code on a network for remote execution. Ascan be envisioned by one of skill in the art, many differentcombinations of the above may be used and accordingly the presentinvention is not limited by the specified scope.

What is claimed is:
 1. A system for performing neurophysiologicassessments during surgery, comprising: a stimulator configured todeliver first and second sets of electrical stimulation signals to themotor cortex of a patient; first and second sensors, each configured todetect at least one motor evoked potential response evoked by the firstand second sets of electrical stimulation signals; and a control unit incommunication with the stimulator and the first and second sensors, thecontrol unit being configured to (a) maintain first and second channels,said first channel associated with said first sensor and said secondchannel associated with said second sensor; (b) in response to a userinput to initiate stimulation, direct transmission of the first andsecond sets of electrical stimulation signals, each of said first andsecond sets of electrical stimulation signals including stimulationsignals having different electrical current amplitude, (c) receiveevoked motor evoked potential response data from the first and secondsensors, (d) determine a first lowest stimulation current amplitude forthe first channel from the first set of electrical stimulation signalsthat evokes a motor evoked potential response greater than a thresholdlevel, (e) determine a second lowest stimulation current amplitude forthe second channel from the second set of electrical stimulation signalsthat evokes a motor evoked potential response greater than saidthreshold level, wherein determining the second lowest stimulationcurrent amplitude is based at least in part on a determination of thelowest stimulation signal amplitude of the first set of stimulationsignals that recruited on the second channel and the highest stimulationsignal amplitude of the first set of stimulation signals that did notrecruit on the second channel; and (f) communicate an onscreenassessment of a spinal cord health status to be displayed to a user inresponse to (d) and (e).
 2. The system of claim 1, wherein each of thefirst and second sets of electrical stimulation signals comprises apredetermined number of pulses separated by an interpulse gap, eachpulse having a pulse width.
 3. The system of claim 2, wherein the numberof pulses ranges from 1to 8 monophasic pulses, the interpulse gap rangesfrom 1 ms to 10 ms, and the pulse width ranges from 50 μs to 400 μs. 4.The system of claim 3, wherein the monophasic pulses are positive phasespulses for all stimulation signals in the set of stimulation signals. 5.The system of claim 2, wherein each of the first and second sets ofelectrical stimulation signals have amplitudes within a range from 0milliamps to 1000 milliamps.
 6. The system of claim 2, wherein thecontrol unit is configured to optimize at least one stimulation signalfrom the first and second sets of electrical stimulation signals atleast one of before conducting a neurophysiologic assessment of thespinal cord and after a response to the stimulation signal stops beingdetected during the neurophysiologic assessment.
 7. The system of claim6, wherein the control unit optimizes said at least one stimulationsignal by modifying at least one of the number of pulses, the interpulsegap, the pulse width, and the current level before the neurophysiologicassessment of the spinal cord.
 8. The system of claim 6, wherein theonscreen assessment of the spinal cord health status is dependent uponat least one of the first lowest stimulation current amplitude and thesecond lowest stimulation current amplitude.
 9. The system of claim 7,wherein the onscreen assessment of the spinal cord health statuscommunicated by the control unit comprises: (i) a numericalrepresentation of at least one of the first lowest stimulation currentamplitude and the second lowest stimulation current amplitude, and (ii)a color identifier to be displayed to a user.
 10. The system of claim 1,wherein the control unit is configured to perform a threshold huntingalgorithm to identify at least one of the lowest first stimulationcurrent amplitude and the second lowest stimulation current amplitude.11. The system of claim 10, wherein the threshold hunting algorithm isbased on successive approximation from different stimulation currentamplitudes.
 12. The system of claim 11, wherein the successiveapproximation involves: (a) establishing a bracket within which at leastone of the first lowest stimulation current amplitude and the secondlowest stimulation current amplitude is contained; and (b) successivelybisecting the bracket until at least one of the first lowest stimulationcurrent amplitude and the second lowest stimulation current amplitude isdetermined within a specified accuracy.
 13. The system of claim 10,wherein the control unit is configured to perform the threshold huntingalgorithm for motor evoked potential responses from multiple musclemyotomes.
 14. The system of claim 1, further comprising a display incommunication with the control unit for displaying the onscreenassessment of the spinal cord health status.
 15. The system of claim 14,wherein the display includes touch-screen control capabilities to allowa user to interface with the control unit.
 16. The system of claim 15,wherein the touch-screen control allows a user to at least one of selectthe parameters of the first and second sets of electrical stimulationsignals and set a reminder to apply the stimulation signal at aspecified time.
 17. The system of claim 15, wherein the display isconfigured to communicate at least one of a baseline motor evokedpotential response threshold, a secondary motor evoked potentialresponse threshold, and the difference between the baseline motor evokedpotential response threshold and the secondary motor evoked potentialresponse threshold.
 18. The system of claim 1, wherein the control unitis further configured to, in response to user input, transition into amanual mode in which the user selects a fixed stimulation currentamplitude for a manual mode stimulation signal.
 19. The system of claim18, wherein the controller, when transitioned to the manual mode, isconfigured to receive the selection of the fixed stimulation currentamplitude for the manual mode stimulation signal and to communicate tothe user whether or not a motor evoked potential response has beendetected for the selected stimulation current amplitude.
 20. The systemof claim 1, comprising a bite-block for placement in the patient'smouth.
 21. The system of claim 20, wherein the bite-block is incommunication with the control unit and the control unit cannot generatea stimulation signal unless the bite-block is positioned within thepatient's mouth.
 22. The system of claim 21, wherein the bite-blockcontains at least one electrode in communication with the control unit.23. The system of claim 1, comprising a second stimulator configured todeliver a third set of electrical stimulation signals to one or morenerves within the patient, the control unit being further configured todirect transmission of the third set of stimulation signals, (b) receiveevoked neuromuscular response data from the first and second sensors inresponse to the third set of stimulation signals, (c) assess a status ofat least one of bone integrity, nerve direction, nerve pathology, andneuromuscular pathway integrity by identifying a relationship between athird lowest stimulation current amplitude from the third setstimulation signals that evokes a neuromuscular response greater than apredefined threshold, and (d) communicate the assessed status of the atleast one of bone integrity, nerve direction, nerve pathology, andneuromuscular pathway integrity to a user.
 24. The system of claim 23,further comprising a display in communication with the control unit fordisplaying the onscreen assessment of the spinal cord health status andthe assessed status of at least one of bone integrity, nerve direction,nerve pathology, and neuromuscular pathway integrity status, wherein thedisplay includes touch-screen control capabilities to allow a user tointerface with the control unit.
 25. The system of claim 24, wherein thetouch-screen control allows a user to selectively switch between any ofa variety of screens associated with each of the functions ofmotor-evoked potential monitoring, bone integrity assessment, nervedirection assessment, nerve pathology assessment, and neuromuscularpathway assessment.
 26. The system of claim 1, wherein the control unitand the stimulator communicate via at least one of wire communicationand wireless communication.
 27. A system for performing neurophysiologicassessment surgery, comprising: a first stimulator configured to deliverfirst and second sets of electrical stimulation signals to the motorcortex of a patient to perform motor-evoked potential monitoring, thefirst set of stimulation signals including a first electricalstimulation signal having a first stimulation current amplitude and asecond electrical stimulation signal having a second stimulation currentamplitude different from the first electrical current amplitude and thesecond set of electrical stimulation signals including a thirdelectrical stimulation signal having a third stimulation currentamplitude and a fourth electrical stimulation signal having a fourthstimulation current amplitude different from the third electricalcurrent amplitude; a second stimulator configured to deliver a third setof electrical stimulation signals to one or more peripheral nerveswithin the patient to perform at least one of somatosensory evokedpotential monitoring, nerve direction assessment, nerve pathologyassessment, and neuromuscular pathway assessment; a processor incommunication with the first and second stimulators and a plurality ofsensors for detecting the responses to the first, second, and third setsof stimulation signals, the processor being configured to (a) directtransmission of the first, second, and third sets of stimulationsignals, wherein at least the first set of stimulation signals istransmitted in response to a user input to initiate stimulation, (b)receive data from the sensors, (c) determine a first lowest stimulationcurrent amplitude from the first set of stimulation signals deliveredfrom the first stimulator that evokes a motor evoked potential responsegreater than a first threshold level, (d) determine a second loweststimulation current amplitude from the second set of stimulation signalsdelivered from the first stimulator that evokes a correspondingneuromuscular response greater than a second threshold level, whereindetermining the second lowest stimulation current amplitude is based atleast in part on a determination of the lowest stimulation signalamplitude of the first set of stimulation signals that recruited and thehighest signal amplitude of the first set of stimulation signals thatdid not recruit, (e) determine a third lowest stimulation currentamplitude from the third set of stimulation signals delivered from thesecond stimulator that evokes a corresponding neuromuscular responsegreater than a third threshold level; and (f) communicate an onscreenassessment of a spinal cord health status to be displayed to a user inresponse to (c) and (d) and an onscreen assessment of at least one ofnerve direction, nerve pathology, and neuromuscular pathway integritystatus to be displayed to a user in response to (e).
 28. The system ofclaim 27, wherein each stimulation signal of the first set ofstimulation signals comprises a predetermined number of pulses separatedby an interpulse gap, each pulse having a pulse width.
 29. The system ofclaim 28, wherein the number of pulses ranges from 1 to 8 monophasicpulses, the interpulse gap ranges from 1 ms to 10ms, the pulse widthranges from 50 μs to 400 μs, and the respective current level fallswithin a range from 0 milliamps to 1000 milliamps.
 30. The system ofclaim 29, wherein the monophasic pulses are positive phase pulses forall stimulation signals of the first set of stimulation signals.
 31. Thesystem of claim 27, wherein the processor is configured to optimize theparameters of the first set of stimulation signals at least one of priorto performing motor evoked potential monitoring and after a response tothe first set of stimulation signals stops being detected during themotor evoked potential monitoring.
 32. The system of claim 31, whereinthe processor optimizes the first set of stimulation signals bymodifying at least one of the number of pulses, the inter pulse gap, thepulse width, and the respective current level for each of thestimulation signals of the first set of stimulation signals beforeperforming motor evoked potential monitoring.
 33. The system of claim27, wherein the onscreen assessment of the spinal cord health status isdependent upon at least one of the first lowest stimulation currentamplitude and the second lowest stimulation current amplitude, and theonscreen assessment of nerve direction, nerve pathology, andneuromuscular pathway integrity is dependent upon the third loweststimulation current amplitude.
 34. The system of claim 27, wherein theprocessor is configured to perform a threshold hunting algorithm toidentify at least one of the first lowest stimulation current amplitudeand the second lowest stimulation current amplitude.
 35. The system ofclaim 34, wherein the threshold hunting algorithm is based on successiveapproximation from different stimulation current amplitudes.
 36. Thesystem of claim 35, wherein the successive approximation involves: (a)establishing a bracket within which at least one of the first loweststimulation current amplitude and the second lowest stimulation currentamplitude is contained; and (b) successively bisecting the bracket untilat least one of the first lowest stimulation current amplitude and thesecond lowest stimulation current amplitude is determined within aspecified accuracy.
 37. The system of claim 35, wherein the processor isconfigured to perform the threshold hunting algorithm for responses frommultiple sites.
 38. The system of claim 27, further comprising a displayin communication with the processor for displaying the onscreenassessment of the spinal cord health status and the onscreen assessmentof the at least one of bone integrity, nerve direction, nerve pathology,and neuromuscular pathway integrity status.
 39. The system of claim 38,wherein the display includes touch-screen control capabilities to allowa user to interface with the processor.
 40. The system of claim 38,wherein the touch-screen control allows a user to at least one of selectthe parameters of the first stimulation signal and set a reminder toapply the first stimulation signal at a specified time.
 41. The systemof claim 38, wherein the display is configured to communicate at leastone of a baseline response threshold, a secondary response threshold,and the difference between the baseline response threshold and thesecondary response threshold.
 42. The system of claim 27, wherein theprocessor is further configured to, in response to user input,transition into a manual mode in which the user selects a fixedstimulation current amplitude for a manual mode stimulation signal fordelivery from the first stimulator for assessment the spinal cord healthstatus.
 43. The system of claim 42, wherein the onscreen assessment ofthe spinal cord health status communicated by the processor comprises:(i) a numerical representation of at least one of the first loweststimulation current amplitude and the second lowest stimulation currentamplitude, and (ii) a color identifier to be displayed to a user. 44.The system of claim 42, wherein the processor, when transitioned to themanual mode, is configured to receive the selection of the fixedstimulation current amplitude for the manual mode stimulation signal andto communicate to the user whether or not a motor evoked potentialresponse has been detected based on the selected stimulation currentamplitude.
 45. The system of claim 27, comprising a bite-block forplacement in the patient's mouth.
 46. The system of claim 45, whereinthe bite-block is in communication with the processor and the processorcannot generate a stimulation signal unless the bite-block is positionedwithin the patient's mouth.
 47. The system of claim 46, wherein thebite-block contains at least one electrode in communication with theprocessor.